XANES pre-edge vs. edge features
Dear list, I was looking at the XANES standard for Fe foil from Hephaestus and I noticed that there was a small, curved feature at the edge (7112 eV), another inflection point at 7116.4 eV, and then the edge step at around 7131 eV. My question is: why is the feature at 7112 eV considered as the edge and not as a pre-edge feature? Are they due to fundamentally different phenomena? I would like to understand where this type of feature comes from so as to be consistent in the current paper that I am writing. I have attached the standard, just in case. Thanks -- Aditya Shivprasad aps202@psu.edu Ph.D Candidate Nuclear Engineering Department Pennsylvania State University
Aditya,
The distinction between "edge" and "pre-edge" is not very clear, either
when looking at a single spectrum or even conceptually.
In broad terms, the main edge is at the energy where the unoccupied
electron levels start - the Fermi energy. For 1s levels, the transition
is to p levels (and for Fe K edge, the 4p level). So, the main edge is at
the energy of the empty 4p levels. This the transition as being to an
atomic level. In a solid (or liquid), the energy levels above the Fermi
level are highly delocalized and spread over many (if not all) atoms in the
systems. Once you get much above the main edge, it's not very easy to
assign transitions to identifiable atomic transitions, or even assign a
good quantum number to them.
Pre-edge features are generally considered to be unoccupied atomic levels
(that is, still assignable to a particular atom, or at least almost so)
below the main edge. For the transition metal K edges (such as Fe), the
main edge is 1s -> 4p. But Fe has many unoccupied 3d levels. For a K
edge to get to transition to these levels, either you need a quadrupole
transition (unlikely, but not impossible), or (more likely) for
bonding/anti-bonding with ligands (typically oxygen) to mix their
p-orbitals with the metal d-orbitals. This hybridization is often called
a ligand field or crystal field. It often gives very identifiable (and
at very predictable energies) peaks below the main edge. Two and
sometimes even three peaks can be seen and assigned with ligand field
terminology. There's sort of a whole industry built up around these
peaks for transition metal oxides.
These peaks can "leak" into the main edge, and in some cases (say, Cu1+)
the classification of "sharp features at the edge" is not very clear. For
Fe metal, it's pretty clear that the main edge (derivative at 7110.75 eV, a
small peak on the main edge around 7112.5 eV) is the 4p level, and the rest
of the features are actually explainable as EXAFS.
Hopefully, someone will correct anything I got wrong!
--Matt
On Mon, Jan 25, 2016 at 8:13 AM, Aditya Shivprasad
Dear list,
I was looking at the XANES standard for Fe foil from Hephaestus and I noticed that there was a small, curved feature at the edge (7112 eV), another inflection point at 7116.4 eV, and then the edge step at around 7131 eV. My question is: why is the feature at 7112 eV considered as the edge and not as a pre-edge feature? Are they due to fundamentally different phenomena? I would like to understand where this type of feature comes from so as to be consistent in the current paper that I am writing. I have attached the standard, just in case.
Thanks -- Aditya Shivprasad
aps202@psu.edu Ph.D Candidate Nuclear Engineering Department Pennsylvania State University
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Dear Aditya,
Matt has, I think sufficiently explained in details about the
pre-edge and its distinction with main energy jump. If my understanding is
correct, and it is possible that I may be wrong as well, the pre-edge peak,
its appearance shape, intensity and energy value can therefore be used by
organometallic researchers to identify and characterize the specific
geometry as well. e.g. tetrahedral geometry in Fe (II). By theoretical
calculations it is possible to validate such values and thus it is possible
to predict the possible geometry of the metal centre (especially with
metals like Fe, I used it earlier).
I hope this will be helpful as well.
Pushkar
On Mon, Jan 25, 2016 at 11:40 PM, Matt Newville
Aditya,
The distinction between "edge" and "pre-edge" is not very clear, either when looking at a single spectrum or even conceptually.
In broad terms, the main edge is at the energy where the unoccupied electron levels start - the Fermi energy. For 1s levels, the transition is to p levels (and for Fe K edge, the 4p level). So, the main edge is at the energy of the empty 4p levels. This the transition as being to an atomic level. In a solid (or liquid), the energy levels above the Fermi level are highly delocalized and spread over many (if not all) atoms in the systems. Once you get much above the main edge, it's not very easy to assign transitions to identifiable atomic transitions, or even assign a good quantum number to them.
Pre-edge features are generally considered to be unoccupied atomic levels (that is, still assignable to a particular atom, or at least almost so) below the main edge. For the transition metal K edges (such as Fe), the main edge is 1s -> 4p. But Fe has many unoccupied 3d levels. For a K edge to get to transition to these levels, either you need a quadrupole transition (unlikely, but not impossible), or (more likely) for bonding/anti-bonding with ligands (typically oxygen) to mix their p-orbitals with the metal d-orbitals. This hybridization is often called a ligand field or crystal field. It often gives very identifiable (and at very predictable energies) peaks below the main edge. Two and sometimes even three peaks can be seen and assigned with ligand field terminology. There's sort of a whole industry built up around these peaks for transition metal oxides.
These peaks can "leak" into the main edge, and in some cases (say, Cu1+) the classification of "sharp features at the edge" is not very clear. For Fe metal, it's pretty clear that the main edge (derivative at 7110.75 eV, a small peak on the main edge around 7112.5 eV) is the 4p level, and the rest of the features are actually explainable as EXAFS.
Hopefully, someone will correct anything I got wrong!
--Matt
On Mon, Jan 25, 2016 at 8:13 AM, Aditya Shivprasad
wrote: Dear list,
I was looking at the XANES standard for Fe foil from Hephaestus and I noticed that there was a small, curved feature at the edge (7112 eV), another inflection point at 7116.4 eV, and then the edge step at around 7131 eV. My question is: why is the feature at 7112 eV considered as the edge and not as a pre-edge feature? Are they due to fundamentally different phenomena? I would like to understand where this type of feature comes from so as to be consistent in the current paper that I am writing. I have attached the standard, just in case.
Thanks -- Aditya Shivprasad
aps202@psu.edu Ph.D Candidate Nuclear Engineering Department Pennsylvania State University
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-- Best Regards, Pushkar Shejwalkar. Post-doctoral -Researcher, Tokyo Engineering University, Tokyo-to Japan
Aditya,
Pushkar is correct, and there are several pretty good papers talking about the geometry and pre-edge features of Fe. The following paper is one of them which I found very useful.
http://pubs.acs.org/doi/abs/10.1021/ja964352a
Guanghui
2016-01-25
Guanghui Zhang, PhD
Senior Research Associate
Department of Chemistry
Illinois Institute of Technology
3300 South Federal Street, Chicago, IL 60616
E-mail: gzhang21@iit.edu; brighton30@126.com
发件人:pushkar shejwalkar
Hi Aditya,
Matt gave a good explanation of what’s responsible for the various features, which was part of your question. Another aspect of your question has to do with the way we use terminology.
(Bruce’s answer, and your response, came in while I was writing this. So it may be somewhat superfluous, but since it’s already mostly written, I figure I’ll pass it along.)
As Matt said, “edge” is a poorly defined term. One meaning of it is that it’s the big rise in absorption that occurs at the start of a XAFS spectrum. That, for example, is the meaning that’s in use when we try to quantify an “edge step” or an “edge jump” so that we can normalize the data. In that sense, if a feature occurs before most of that rise, it can be called “pre-edge.”
But another use of the word “edge” is as a synonym for “E0,” which is itself a concept that is not unambiguously defined. One definition of E0 would be “the energy origin needed to make the EXAFS equation accurate.” But the EXAFS equation is itself an approximation which is only useful at energies starting many eV above E0. So there’s no theoretical reason that the E0 used in EXAFS has to correspond to an energy with a simple theoretical meaning or to any particular feature in the spectrum. The best we can say is that it should be within, or sometimes a bit above, the rising portion of the absorption spectrum.
But when we’re trying to align and process data we need some working definition of E0, both for XANES analysis and to get a preliminary chi(k) for EXAFS analysis. So we come up with other definitions, such as “the maximum of the first significant peak in the first derivative spectrum.” As Bruce said, the exact definition used is not important, but it is important that the same definition be used with all spectra being compared.
Depending on the definition chosen, the preliminary value of E0 may very well be chosen well below the bulk of the rise in the spectrum. If we call the energy of E0 the “edge,” but also call features that come before the big rise "pre-edge features,” we end up with the confusing terminology that the edge energy may well be below some of the pre-edge features!
That terminology is unfortunate for people new to the field, but rarely causes any actual ambiguity.
—Scott Calvin
Sarah Lawrence College
On Jan 25, 2016, at 9:40 AM, Matt Newville
On 01/25/2016 09:13 AM, Aditya Shivprasad wrote:
I was looking at the XANES standard for Fe foil from Hephaestus and I noticed that there was a small, curved feature at the edge (7112 eV), another inflection point at 7116.4 eV, and then the edge step at around 7131 eV. My question is: why is the feature at 7112 eV considered as the edge and not as a pre-edge feature? Are they due to fundamentally different phenomena? I would like to understand where this type of feature comes from so as to be consistent in the current paper that I am writing. I have attached the standard, just in case.
Hi Aditya, That's a great question! In Athena, you are looking to identify the "threshold" or the "zero of wavenumber" as the value for E0. In a metal, this is probably not hard to identify. In a compound, which might have localized, unoccupied states, the question is a bit harder. As a practical matter, you want to process your data consistently and in a way that yields defensibly normalized data so that you can use XANES methods (e.g. LCF or PCA) and EXAFS analysis in a defensible way. As a practical matter -- almost any point on the main rising edge tends to be adequate for either XANES or EXAFS analysis. I was going to go on and talk about how to think about the Fermi energy, but I was going to say basically the same thing that Matt said in his email that arrived while I was typing. Matt's comment about the "XANES industry" is quite right and I encourage you to read up on that. But as a practical matter, these choices should be consistent such the normalization is done in a consistent manner. What do I mean by "consistent"? Well, I believe you said you are doing some kind of LCF analysis of your data. Well, if you were to make a synthetic sample with known quantities of Iron standards, then your methodology for the analysis of the standards and the synthetic sample should yield the same ratios that were used to make the sample. HTH, B -- Bruce Ravel ------------------------------------ bravel@bnl.gov National Institute of Standards and Technology Synchrotron Science Group at NSLS-II Building 535A Upton NY, 11973 Homepage: http://bruceravel.github.io/home/ Software: https://github.com/bruceravel Demeter: http://bruceravel.github.io/demeter/
participants (6)
-
Aditya Shivprasad
-
Bruce Ravel
-
Guanghui Zhang
-
Matt Newville
-
pushkar shejwalkar
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Scott Calvin